Helical springs are machine elements required in large quantities and different configurations in numerous fields of application. Helical springs, which are also referred to as wound torsion springs or coil springs, are normally produced from spring wire and are in the form of tension springs or compression springs depending on their load during use. Compression springs, in particular bearing springs, are required, for example, in large quantities for automobile construction. The spring characteristic can be influenced, inter alia, by sections of different pitch or with different pitch profiles. For example, in the case of compression springs, there is frequently a central section of greater or lesser length with a constant pitch (constant section), adjacent to which, at both ends of the spring, there are contact areas with a pitch which becomes less towards the ends. In the case of cylindrical helical springs, the spring diameter is constant over the length of the springs, but it may also vary over the length, for example, in the case of conical or barrel-shaped helical springs. In addition, the overall length of the (unloaded) spring may vary widely for different applications.
Nowadays, helical springs are normally produced by spring winding with the aid of numerically controlled spring winding machines. In this case, a wire (spring wire) is fed, controlled by an NC control program, by a feed device to a forming device of the spring winding machine, and formed with the aid of tools of the forming device, to form a helical spring. The tools generally include one or more variable-position winding pins to fix and possibly to vary the diameter of spring turns and one or more pitch tools, which govern the local pitch of the spring turns in each phase of the manufacturing process.
Spring winding machines are generally intended to produce a large number of springs with a specific spring geometry (nominal geometry) within very narrow tolerances, at a high rate. The functionally important geometry parameters include, inter alia, the overall length of the finished helical spring in the unloaded state. The overall length also governs, inter alia, the installation dimensions of the spring and the spring force.
To comply with stringent quality requirements, for example, in the automobile field, it is normal practice to measure certain spring geometry data, for example, the diameter, length, pitch, and/or pitch profile of the spring after completion of a spring, and to automatically sort the finished springs, depending on the result of the measurement, into satisfactory parts (spring geometry within the tolerances) and unsatisfactory parts (result outside the tolerances), and possibly into further categories. This procedure is highly uneconomic, in particular in the case of long springs, since, in the case of long springs, a relatively great length of wire is consumed for each spring and must be thrown away if it is found that the finished spring is outside the tolerances.
It has also already been proposed for the diameter, the length and the pitch of the spring to be checked by suitable measurement means during manufacture, and for manufacturing parameters to be changed in the event of any discrepancies outside the tolerance limits such that the spring geometry remains within the tolerances. DE 103 45 445 B4 discloses a spring winding machine which has an integrated measurement system with a video camera which is directed at that area of the spring winding machine in which the forming of the spring starts. An image processing system connected to the video camera and having appropriate evaluation algorithms is intended to allow the diameter, length and pitch of the spring to be checked during manufacture, and it is intended to be possible to vary these spring geometry parameters by feedback to the processing tools, which can be adjusted by motors, during manufacture. An evaluation algorithm for determining the current spring diameter is described in detail.
It could therefore be helpful to provide a method and an apparatus of a generic type such that, particularly when producing relatively long helical springs helical springs can be produced within tight geometric tolerances with high reliability, composed of wire materials of widely differing quality. It could also be helpful to provide for the production of long helical springs with little overall length scatter and with a low scrap rate.